An essential component of a compression ignition internal combustion engine, hereinafter to be referred to as a diesel engine, is a high pressure fuel injector pump, the moving parts of which have hitherto been lubricated by the diesel fuel passing through it. It has long been observed that winter grades of diesel fuel, which are of lower viscosity and contain less waxy fractions than summer grade diesel fuels, have poorer load bearing capacity, or are less capable of lubricating the moving parts of the injector pump, i.e. have poorer lubricity. Recent European regulations have reduced the sulphur content in diesel fuel to 0.05 wt %, and ultra-low sulphur fuels are also now available containing only 0.001 wt % sulphur. Although environmentally beneficial, the reduction in sulphur content has certain disadvantages. In particular, it reduces the lubricity of the diesel fuel, and as a result problems have been reported with excessive wear, including the premature failure of the load bearing parts of certain manufactures of injector pump.
This problem has been addressed by adding to the fuel additives which impart anti-wear properties and improved lubricity to the fuel. For example U.S. Pat. No. 4,849,119 discloses fuels containing friction-reducing additives which comprise a diamine dicarboxylate, made by reacting together a diamine and an organic monocarboxylic acid. The diamine has the formula RR.sup.1 N--R.sup.2 --NR.sup.3 R.sup.4, where R, R.sup.1, R.sup.3 and R.sup.4 are H or C.sub.6 -C.sub.20 hydrocarbyl, and R.sup.2 is C.sub.2 -C.sub.4 hydrocarbylene. It is stated that the hydrocarbyl group may also be aryl, although no examples are given.
EP-A-798364 discloses as lubricity additives for diesel fuel salts of a carboxylic acid and an aliphatic amine.
It has now been discovered that salts of certain carboxylic acids and aromatic heterocyclic amines are unexpectedly effective in enhancing the lubricity of hydrocarbon fuels, especially diesel fuels.